Structural And Adsorptive Characteristics
Structural and Adsorptive Characteristics
of Fumed Titania/Silica and Alumina/Silica/Titania
at High Concentration of Titania
V.M. Gun\’ko1, I.F. Mironyuk1, V.I. Zarko1, E.V. Goncharuk1,
M.V. Borysenko1, R. Leboda2, J. Skubiszewska-Zięba2,
B. Charmas2, W. Janusz2, and S. Chibowski2
1Institute of Surface Chemistry, Ukrainian National Academy of Sciences, General Naumov st. 17, 03164 Kyev, Ukraine
2Faculty of Chemistry, Maria Curie-Sklodowska University,
Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland
Mixed oxides with titania, silica and alumina are widely used as catalysts, pigments, fillers, adsorbents, metal catalyst carrier, etc. For individual titania (anatase, rutile or their blend), binary (titania/silica, TS, alumina/titania) and ternary (alumina/silica/titania, AST) oxides, variations in the synthesis temperature, ratios between reactant concentrations (e.g., MCln, O2, H2 and their distributions in the flame and flow velocity), etc. allow one to synthesize materials possessing different morphology, rutile/anatase ratio, active surface site distribution, adsorptive properties, etc. Previously, we studied fumed titania/silica, CVD-titania/fumed silica or CVD-titania/silica gel at lower concentration of titania (CTiO2 CSiO2 (or CAl2O3) the properties of oxide as the whole can depend on the characteristics of titania stronger than on the second oxide (depending on their surface concentrations) that is of importance on application of fumed mixed oxides containing titania. Therefore the aim of this work was to synthesize and explore fumed titania/silica and alumina/silica/titania at CTiO2 ³ 50 wt.% under varied conditions.
Fumed TS and AST synthesized at a large concentration of titania (³50wt.%) were studied by means of XRD, photon correlation spectroscopy, electrophoresis, and adsorption methods. Explored TS and AST consist of amorphous silica and alumina and crystalline titania (blend of anatase and rutile). The shapes of adsorption isotherms and aS plots show the main contribution of mesopores (channels or gaps between primary particles in aggregates) to the total porosity. The samples at CTiO2 > 80 wt.% are characterized
Table 1.Structural Parameters of Fumed Titania/Silica and Alumina/Silica/Titania
Sample
CTiO2
wt. %
CSiO2
wt. %
CAl2O3
wt. %
g
SBET
M2/g
Vp
cm3/g
Dp
nm
rap
g/L
D
nm
Fractal DAJ
TS6
94
6
-
0.79
30
0.077
10.1
137
51
2.500
TS35
65
35
-
0.84
34
0.065
7.7
166
52
2.552
TS37
63
37
-
1.38
83
0.174
8.4
40
22
2.517
AST50
50
28
22
7.33
38
0.070
15.4
51
2.280
AST71
71
8
21
2.46
74
0.105
5.7
81
24
2.584
AST82
82
6
12
0.83
39
0.106
10.8
115
42
2.525
AST87
87
4
9
0.91
42
0.118
11.2
56
38
2.530
AST88
88
8
4
0.93
39
0.091
9.3
95
41
2.538
Note. g = Canatase/Crutile; Vp is the pore volume; Dp is the average pore diameter; rap is the apparent density; D is the average diameter of primary particles.
by lower SBET and Vp and greater rap. Typically, the smaller the flow velocity (vf) in the flame, the larger the primary particles (as follows D~1/ln vf), while they are in the flame during longer time of oxide formation. Increase in the size of primary particles leads to decrease in the empty space in aggregates, which is close to Vp (estimated from the nitrogen adsorption at p/p0» 0.98), while the empty space in powder Vem = (1000/rap - rap/r0) » 6-25cm3/g is significantly larger than Vp. Consequently, structural features of fumed oxides at different levels of their structural multistep hierarchy depend strongly on the characteristics of primary particles. Large amounts of silica (> 60 wt.%) can promote formation of anatase and inhibition of its transition to rutile at T higher than the temperature of change of phase, but in these experiments g
Zeta potential of AST samples as a function of pH demonstrates a marked influence of the alumina phase at pH 8. The particle size distribution in the diluted (Cox = 0.04 wt.%) aqueous suspensions of TS and AST depends nonlinearly on pH due to balance of attractive and repulsive interactions of the surface patches with different compositions. The effective diameter of particle swarms depends on pH between IEPTiO2 and IEPAl2O3 stronger for AST than for TS. AST is characterized by a greater adsorptive ability in respect to proteins (BSA) and metal cations (Pb(II)) than corresponding individual and binary oxides. The z(pH) curve shifts toward negative z values and slightly changes its shape with increasing CSiO2. In the case of AST, over the pH range corresponding to low charge density on titania (isoelectric point (IEP) at » 6), z(pH) of AST is close to that of alumina. However, at pH close to pH(IEP) of alumina (»9.8), the z(pH) curves have the larger decline than alumina due to the impact of titania and silica phases. On Pb(II) uptake, increase in its concentration leads to the displacement of the plateau adsorption toward pH > pHPZC(TiO2) » 5.5-6.0; i.e., the uptake of aqueous Pb(II) grows with appearance of negative charge on the titania phase, as PZC of fumed silica lies at pH » 2.2 and at »9.8 for fumed alumina. For mixed oxides such as TS and AS at CSiO2 > 50 wt.%, pHPZC (or IEP) lie at pH
Acknowledgment
This research was supported by NATO (grant No. EST.CLG.976890), the Polish State Committee for Scientific Research and Ministry of High Education and Science of Ukraine (grant No. 2М/303-99).
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